新疆西准噶尔库什库都克岩体地质时代、地球化学及岩石成因

Geological age, geochemistry and petrogenesis of Kushikuduke granite in western Junggar, Xinjiang

新疆西准噶尔构造带分布着大量中酸性侵入体，其性质和侵位期次对于认识该构造带的演化及其成矿过程具有十分重要的意义。然而，由于部分岩体缺乏精确的同位素年代学和地球化学数据（如库什库都克岩体），制约了对其成因类型的认识。本文通过对库什库都克岩体野外地质调查、LA—ICP-MS锆石U—Pb同位素测年及岩石地球化学分析，探讨其形成时代和成因类型。结果表明，该岩体出露面积仅约为3km2，岩性主要为花岗闪长岩，见少量花岗闪长斑岩。LA—ICP—MS锆石U—Pb获得其形成时代为311．9±2．6Ma（n=20，MSWD=0．05）。地球化学特征显示其具有I型花岗岩的特征。岩体主量元素具有相对富硅（SiO2=65．78％-67．76％）、中等铝（Al2O3=15．66％-16．29％）、富碱（Na2O＋K2O=7．79％-8．04％）、贫钾（K2O=3．48％-3．86％）的特征，FeOT／MgO值较低（3．77—4．36），A／CNK=0．99—1．20，A／NK=1．43～1．48，属高钾钙碱性准铝质-弱过铝质岩石；稀土配分曲线为右倾型，负Eu异常较明显（δEu=0．57-0．72），微量元素蛛网图显示富集大离子亲石元素Rb、Ba，相对亏损Nb、Ta、Ti等高场强元素和重稀土Yb、Y，10^4Ga／A1值较低（2．21—2．56）等特征。这与区内如包古图、别鲁阿嘎希等钙碱性系列的小岩体在成岩时代、地球化学特征以及成因类型上具有相似性，表明它们可能具有相同的地球动力学背景，同为俯冲作用的产物。而与300Ma左右形成于后碰撞环境的庙尔沟、阿克巴斯陶等A2型大岩基明显不同，暗示西准噶尔地区可能存在着性质不同的两期岩浆活动，这为区域晚古生代岩浆活动和构造演化等研究提供了资料。

There are large numbers of intermediate-acid intrusions exposed along the tectonic- magmatism belt of western Junggar, Xinjiang. It is of great significance to know the evolution and mineralization process of this tectonic-magmatism belt through their nature and emplacement periods. However, there arc still lack of accurate isotopic chronology and geochemical datas of part of the small rock bodies to restricting the understanding of their genetic types, including Kushikuduke granite. The petrogenic age and genetic type are discussed in this paper through the field observation, LA-ICP-MS zircon U-Pb isotopic dating and geochemical analysis. The results show that Kushikuduke granite with an outcrop area of 3 km2, and is mainly composed of granodiorite and small amount of granodiorite-porphyry. LA-ICP-MS zircon U-Pb dating reveals that the Kushikuduke granite formed at 311.9±2.6 Ma （n =20, MSWD = 0.05）, which belongs to the Early Late Carboniferous epoch. Petrological and geochemical studies show that Kushikuduke granite belongs to the I-type granite, which is characteristic of relatively abundant of Si （SIO2=65.78% ~ 67.76% ）, medium Al （Al2O3 = 15.66% -16.29% ）, abundant alkali （Na2O＋ K2O=7.79% -8.04% ） and depleted K （K2O = 3.48% - 3.86% ）, and with low value of the FeOT/MgO （3.77 - 4.36）, A/CNK - 0.99 - 1.20, A/ NK-1.43- 1.48, which belongs to high-K calc-alkaline, metaluminous to peraluminous series. The chondrite-normalized REE pattern plots displayed as the rightist type, and negative Eu anomalies with low δEu （δEu = 0.57 - 0.72）, the primitive mantle-normalized trace elements spider plots are characterized by enrichment in large-ion lithophile elements （LILE） such as Rb and K, and depletion in high field strength elements （HFSE） such as Nb, Ta, Ti, and heavy rare-earth elements （HREE） such as Y and Yb, with low value of the 104 Ga/Al （2.21 - 2.56, the averaging 2.43）, which indicate that it is different from A- or S-type granite, suggesting that Kushikuduke granite is actually typical metaluminous to peraluminous I-type granite. These results are extremely similar to other small intrusions such as Baogutu, Bieluagaxi and Dulunhe granite of western Junggar in the petrogenic age, geochemical characteristics and genetic types, which demonstrates that these small intrusions may have the same geodynamic background, and all of them are the products of subduction, and is markedlydifferent from large batholiths such as Miaoergou and Akebasitao granite, which belong to the A2-type granite formed in an post-collisional tectonic setting approximately at 300 Ma, suggesting that it may exist two periods and distinctive nature of characteristics of magmatic activities of western Junggar, and provides new datas for the research of magmatic activities and tectonic evolutions of the Late Paleozoic.